A packet communication system, similar to other types of communication systems, provides for the communication of data between communication stations within a set of communication stations. The set includes at least a sending station and a receiving station. Data originated at, or otherwise provided to, a sending station is caused to be communicated by the sending station for delivery at a receiving station. The data from the sending station is sent through a communication channel, and the receiving station monitors the communication channel, thereby to detect delivery of the data communicated thereon.
In a packet communication system, data that is communicated is first packetized into packets of data, and the data packets, once formed, are then communicated, sometimes at discrete intervals. Once delivered to a receiving station, the information content of the data is ascertained by concatenating the information parts of the packets together. Packet communication systems generally make efficient use of communication channels as the communication channels need only to be allocated pursuant to a particular communication session only for the period during which the data packets are communicated. Packet communication channels are sometimes, therefore, shared communication channels that are shared by separate sets of communication stations between which separate communication services are concurrently effectuated.
Operating specifications that define the operating protocols of various types of packet radio communication systems have been promulgated and yet others are undergoing development and standardization. A packet radio communication system provides the advantages of a radio communication system in that the communication stations that are parties to a communication session need not be interconnected by electrically-conductive connectors. Instead, the communication channels of a packet radio communication system are formed of radio channels, defined upon a portion of the electromagnetic spectrum.
While packet radio communication systems have been developed for the effectuation of various different types of communication services, much recent interest has been directed towards the development of packet radio communication systems capable of providing data-intensive communication services. For instance, the IEEE 802.15.3a operating specification contemplates an Orthogonal Frequency Division Multiplexing (OFDM) Ultra Wide Band (UWB) communication system, capable of communicating data over wide bandwidths over short ranges.
Ultra Wideband (UWB) is defined as any radio technology having a spectrum that occupies a bandwidth greater than 20 percent of the center frequency, or a bandwidth of at least 500 MHz. Modern UWB systems use modulation techniques, such as Orthogonal Frequency Division Multiplexing (OFDM), to occupy these extremely wide bandwidths.
OFDM is a special case of frequency division multiplexed transmission that permits subcarriers to overlap in frequency without mutual interference, thereby increasing spectral efficiency. OFDM used for UWB transmission results in a novel physical layer system for the enablement of high bit rate, short-range communication networks.
The information transmitted on each band is modulated using OFDM, which distributes the data over a large number of carriers that are spaced at precise frequencies. This spacing provides the orthogonality in this technique, which prevents interference from adjacent tones. The benefits of OFDM include high-spectral efficiency, resiliency to radio frequency (RF) interference, and lower multipath distortion.
A structured data format is set forth in the present promulgation of the operating specification. The data format of a data packet formed in conformity with the IEEE 802.15.3a includes a preamble part and a payload part. Other packet communication systems analogously format data into packets that also include a preamble part and a payload part. The payload part of the packet contains the information that is to be communicated. That is to say, the payload part is nondeterminative. Conversely, the preamble part of the data packet does not contain the informational content that is to be communicated but, rather, includes determinative data that is used for other purposes. In particular, the preamble part of an IEEE 802.15.3a packet preamble includes three parts, a packet sync sequence, a frame sync sequence, and a channel estimation sequence. The packet sync sequence is of a length of twenty-one OFDM (symbols), the frame sync sequence is of a length of three OFDM symbols, and the channel estimation sequence is of a length of six OFDM symbols. Collectively, the sequences are of a time length of 9.375 microseconds.
The preamble portions are used, for instance, to facilitate synchronization between the sending and receiving stations that send and receive the data packet, respectively. Additionally, the preamble is used for purposes of automatic gain control (AGC). For use of automatic gain control, a receiving station is able to set its gain at an appropriate level, e.g., to facilitate application of received data to an analog-to-digital converter to supply useful bits to a baseband part of the receiving station.
The preamble is further used for purposes of packet detection. Packet detection identifies to the receiving station the reception at the receiving station of the packet. Upon detection of the packet, e.g., various state machines at the receiving station are started to enable processing of the incoming packet.
In packet-based wireless communications systems, the transmitting device must decide at what data rate to transmit each packet. It is generally the case that higher rates yield lower robustness and thus increase the probability that the packet will not be decodable at the receiver. However, it is desirable to transmit at higher data rates when possible. Thus, the transmitting device typically attempts to transmit at the highest data rate possible while still maintaining an acceptable probability of error at the receiver. The transmitter typically implements an algorithm that attempts to select the proper transmission rate that maximizes the throughput of the system. This algorithm is referred to as the rate control algorithm.
Present packet-based wireless systems, such as IEEE 802.11 or IEEE 802.15 systems, typically have a rate control algorithm that is run in an open loop (i.e. the transmitter has no feedback from the receiver as to which transmission rate is best). Additionally, the rate control algorithm itself is typically left outside the scope of the standard (i.e. left up to the implementer). In today's systems, rate control algorithms reduce the data rate when a packet error occurs. An error is typically identified by the absence of an acknowledgment (ACK) packet from the receiver. The rate is increased in a heuristic fashion (e.g., the system shifts to the next higher data rate after some period of time without significant errors).
Therefore, it would be desirable to have an improved method of controlling the data rate for packet-based communications.